drm/panel: sony-acx565akm: Use backlight helpers

drm/panel: sony-acx565akm: Use backlight helpers

Instead of retrieving the backlight brightness in struct
backlight_properties manually, and then checking whether the backlight
should be on at all, use backlight_get_brightness() which does all
this and insulates this from future changes.

Instead of manually checking the power state in struct
backlight_properties, use backlight_is_blank().

While we're at it, drop .fb_blank from the initialisation function; it
is deprecated, and this helps make progress towards enabling its
removal. This change makes no functional difference since
FB_BLANK_UNBLANK is the default value.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220616172316.1355133-4-steve@sk2.org